Packaged Semiconductor Device with Encapsulant Embedding Semiconductor Chip that Includes Contact Pads

ABSTRACT

A method of manufacturing a semiconductor package includes embedding a semiconductor chip in an encapsulant. First contact pads are formed on a first main face of the semiconductor package and second contact pads are formed on a second main face of the semiconductor package opposite the first main face. A diameter d in micrometers of an exposed contact pad area of the second contact pads satisfies d≧(8/25)x+142 μm, where x is a pitch of the second contact pads in micrometers.

This is a divisional application of U.S. application Ser. No.12/750,946, which was filed on Mar. 31, 2010, and is incorporated hereinby reference.

TECHNICAL FIELD

The invention relates generally to semiconductor packages, and moreparticularly to a semiconductor package configured for use in a stackedpackage device.

BACKGROUND

Market demand for smaller and more functional electronic devices hasdriven the development of semiconductor devices, including semiconductorpackages, and entire systems including multichip packages or stacks ofpackages. The space available inside the electronic devices is limited,particularly as the electronic devices are made smaller. Stackedpackages, especially package-on-package (PoP) techniques are oneapproach in today's packaging technology to cope with reduced boardspace.

Both the manufacturers and the consumers of electronic devices desiredevices that are reduced in size and yet have increased devicefunctionality.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIGS. 1A to 1F schematically illustrate one exemplary embodiment of amethod of manufacturing a semiconductor package;

FIGS. 2A to 2E schematically illustrate one exemplary embodiment of amethod of manufacturing a semiconductor package;

FIG. 3 schematically illustrates one embodiment of a semiconductorpackage;

FIG. 4 schematically illustrates a package-on-package device showingwarpage and contact failure;

FIG. 5 schematically illustrates a semiconductor package having a firstcontact pad area of conventional diameter and a second contact pad areaof enlarged diameter;

FIG. 6 schematically illustrates a package-on-package device showingwarpage without contact failure;

FIG. 7 schematically illustrates one embodiment of a semiconductorpackage; and

FIG. 8 schematically illustrates one embodiment of a semiconductorpackage.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” “upper,” “lower,”etc., is used with reference to the orientation of the figure(s) beingdescribed. Because components of embodiments can be positioned in anumber of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

As employed in this specification, the terms “coupled” and/or“electrically coupled” are not meant to mean that the elements must bedirectly coupled together; intervening elements may be provided betweenthe “coupled” or “electrically coupled” elements.

Packages and devices with semiconductor chips are described below. Thesemiconductor chips may be of different types, may be manufactured bydifferent technologies and may include for example, integratedelectrical, electro-optical or electro-mechanical circuits and/orpassive devices. The semiconductor chips may, for example, be designedas logic integrated circuits, analog integrated circuits, mixed signalintegrated circuits, memory circuits or integrated passive devices. Theymay include control circuits, microprocessors or microelectromechanicalcomponents. By way of example, a semiconductor chip may be a basebandchip used in mobile communications devices. Further, they may includepower semiconductor devices, such as power MOSFETs (Metal OxideSemiconductor Field Effect Transistors), IGBTs (Insulated Gate BipolarTransistors), JFETs (Junction Gate Field Effect Transistors), powerbipolar transistors or power diodes. In particular, semiconductor chipshaving a vertical structure may be involved, that is to say that thesemiconductor chips may be fabricated in such a way that electriccurrents can flow in a direction perpendicular to the main faces of thesemiconductor chips. A semiconductor chip having a vertical structuremay have contact elements, in particular, on its two main faces, that isto say on its top side and bottom side. In particular, powersemiconductor chips may have a vertical structure. By way of example,the source electrode and gate electrode of a power MOSFET may besituated on one main face, while the drain electrode of the power MOSFETis arranged on the other main face. Furthermore, the devices describedbelow may include integrated circuits to control the integrated circuitsof other semiconductor chips, for example, the integrated circuits ofpower semiconductor chips. The semiconductor chips need not bemanufactured from specific semiconductor material, for example, Si, SiC,SiGe, GaAs, AlGaAs and, furthermore, may contain inorganic and/ororganic materials that are not semiconductors, such as, for example,insulators, plastics or metals.

The devices described below include external contact pads on thepackage. The contact pads may represent external terminals of thepackage. They may be accessible from outside the package and may thusallow electrical contact to be made with the semiconductor chips fromoutside the package. Furthermore, the (external) contact pads may bethermally conductive and may serve as heat sinks for dissipating theheat generated by the semiconductor chip. The contact pads may becomposed of any desired electrically conductive material, for example,of a metal, such as copper, aluminum or gold, a metal alloy or anelectrically conductive organic material. Solder material, such assolder balls or solder bumps, may be deposited on the external contactpads.

The semiconductor chips, or at least parts of the semiconductor chips,are covered with an encapsulant which may be electrically insulating.The encapsulant may be a dielectric material and may be made of anyappropriate duroplastic, thermoplastic or thermosetting material orlaminate (prepreg). The encapsulant may contain filler materials. Afterits deposition, the encapsulant may be only partially hardened and maybe completely hardened after application of energy (e.g., heat, UVlight, etc.). Various techniques may be employed to cover thesemiconductor chips with the encapsulant, for example, compressionmolding, injection molding, powder molding, liquid molding, dispensingor laminating.

The encapsulant may be used to produce so-called fan-out type packages.In a fan-out type package, at least some of the external contact padsand/or conductor tracks electrically connecting the semiconductor chipto the external contact pads are located laterally outside of theoutline of the semiconductor chip or at least intersect the outline ofthe semiconductor chip. Thus, in fan-out type packages, a peripherallyouter part of the package of the semiconductor chip is typically(additionally) used for electrically bonding the package to externalapplications, such as, e.g., application boards or, in stacked packageapplications, other packages. This outer part of the packageencompassing the semiconductor chip effectively enlarges the contactarea of the package in relation to the footprint of the semiconductorchip, thus leading to relaxed constraints in view of package pad sizeand pitch with regard to later processing, e.g., second level assembly.

Portions of the encapsulant may be removed, for example, in order tocreate one or more recesses, through-holes or trenches in theencapsulant. Removing the encapsulant may be carried out by using alaser beam or a water jet, mechanical sawing using a saw or a cutter,chemical etching, milling or any other appropriate method. In therecesses, through-holes or trenches, electrically conductive materialmay be deposited, for example, in order to create one or morethrough-connections. The through-connections may extend from a firstface of the encapsulant to a second face of the encapsulant. Thethrough-connections are electrically conductive and may electricallycouple an electrically conductive layer on the first face to anelectrically conductive layer on the second face of the encapsulant ofthe package. The through-connections may, for example, be vias (verticalinterconnect access).

The recesses, through-holes or trenches may, for example, be filled witha paste containing metal particles. The metal particles may, forexample, be made of silver, gold, copper, tin or nickel. The metalparticles may be dispersed in a suitable liquid or solvent. After theirapplication, the metal particles may be heated and thereby sintered.Apart from the recesses, through-holes and trenches, the metal particlesmay be deposited onto any other surface of the encapsulant.

One or more metal layers may be placed over the encapsulant and/or thesemiconductor chip embedded in the encapsulant. The metal layers may,for example, be used to produce a redistribution layer within aconductive redistribution structure. The metal layers may be used aswiring layers to make electrical contact with the semiconductor chipsfrom outside the package and/or to make electrical contact with othersemiconductor chips and/or components contained in the package. Themetal layers may be manufactured with any desired geometric shape andany desired material composition. The metal layers may, for example, becomposed of conductor tracks, but may also be in the form of a layercovering an area. They may be used to provide the contact pads of thepackage. Any desired metal, for example, aluminum, nickel, palladium,silver, tin, gold or copper, or metal alloys may be used as thematerial. The metal layers need not be homogenous or manufactured fromjust one material, that is to say various compositions andconcentrations of the materials contained in the metal layers arepossible. Thin-film technologies may be applied to generate and/orstructure the metal layers. Furthermore, the metal layers may bearranged above or below or between electrically insulating layersforming part of the conductive redistribution structure. An insulatinglayer overlaying a metal layer may be used as a solder stop of thecontact pads.

FIGS. 1A to 1F schematically illustrate a method of manufacturing asemiconductor package 100 (see also FIG. 3). In a first step (FIG. 1A),a (temporary) carrier 1 is provided. The carrier 1 may be rigid or maybe flexible to a certain degree and may be fabricated from materialssuch as metals, metal alloys, silicon, glass or plastics. An adhesivetape 2 may be laminated on the carrier 1. The adhesive tape 2 may be adouble sided sticky tape. Alternatively, a glue material or any otheradhesive material or mechanical securing means (such as a clampingdevice or a vacuum generator) may be associated with the carrier 1.

At least two semiconductor chips 10 are provided and placed on thecarrier 1. Further, as shown in FIG. 1A, via bars 11 may be placed onthe carrier 1 in spacings between adjacent semiconductor chips 10. Thevia bars 11 may include an envelope 11 a which may be made of aninsulating material and at least one conducting element 11 b runningthrough the envelope 11 a. By way of example, PCB (printed circuitboard) via bars 11 may be used. In PCB via bars 11, the insulatingmaterial of the envelope 11 a may be made of epoxy resin. Thesemiconductor chips 10 and the via bars 11 are fixed on the carrier 1 bymeans of the adhesive tape 2 or other appropriate equipment.

The distance between neighboring semiconductor chips 10 may be in therange of between 200 μm and 10 mm. It is to be noted that throughoutFIGS. 1A to 1F, only a partial section of a semiconductor chip array or“artificial wafer” is illustrated, that is to say in practice, typicallymany more than two semiconductor chips 10 are placed on the carrier 1.

Semiconductor chips 10 may have contact elements 14, 15 on a lower mainchip surface 12 facing the carrier 1. If the semiconductor chip 10 is alogic integrated circuit, typically several tens of contact elements 14,15 are arranged on the lower main chip surface 12. The lower main chipsurface 12 typically forms the active surface of the semiconductor chip10. If the semiconductor chips 10 are power transistors, the contactelement 14 may, e.g., be a source terminal and the contact element 15may, e.g., be a gate terminal.

An electrically insulating molding material or encapsulant 18 is appliedto the semiconductor chips 10 and the carrier 1, see FIG. 1B. Theencapsulant 18 may be used to encapsulate the semiconductor chips 10except their lower main chip surfaces 12 containing the contact elements14, 15. The encapsulant 18 may be an epoxy or another appropriatematerial used in contemporary semiconductor packaging technology. It mayalso be a photoresist such as SU8, which is epoxy-based. The encapsulant18 may be composed of any appropriate thermoplastic or thermosettingmaterial. After curing, the encapsulant 18 provides stability to thearray of semiconductor chips 10 and via bars 11. Various techniques maybe employed to cover the semiconductor chips 10 and via bars 11 with theencapsulant 18, for example, compression molding or injection molding.

By way of example, in a compression molding process the liquidencapsulant 18 is dispensed into an open lower mold half of which thecarrier 1 forms the bottom. Then, after dispensing the liquidencapsulant 18, an upper mold half is moved down and spreads out theliquid encapsulant 18 until a cavity between the carrier 1 forming thebottom of the lower mold half and the upper mold half is completelyfilled. This process may be accompanied by the application of heat andpressure. After curing, the encapsulant 18 is rigid and forms a moldedbody. The larger the lateral size of the molded body (“moldedreconstituted wafer”) and the number of embedded chips 10, the more costefficient the process will typically be.

As may be seen from FIG. 1B, the semiconductor chips 10 may becompletely overmolded, i.e., completely covered by encapsulant 18.

In a subsequent step, the encapsulant 18 is released from the carrier 1.To this end, the adhesive tape 2 may feature thermo-release properties,which allow the removal of the adhesive tape 2 during a heat treatment.The removal of the adhesive tape 2 from the encapsulant 18 including thesemiconductor chips 10 and via bars 11 is carried out at an appropriatetemperature which depends on the thermo-release properties of theadhesive tape 2 and is usually higher than 150° C., in particular,approximately 200° C. The encapsulant 18 is also referred to as an“artificial wafer” or “reconstituted wafer” in the art. The encapsulant18 may, e.g., be disc-shaped having a diameter of, e.g., 200 or 300 mm,or may have any other shape such as a polygonal shape and the same orother lateral dimensions.

Further to FIG. 1C, the molded body is thinned (FIG. 1C). Thinning isusually performed after the release of the encapsulant 18 from thecarrier 1. Grinding or lapping machines may be used that are similar oridentical to the machines used for semiconductor wafer grinding orlapping in frontend technology. Alternatively, etching may be used toreduce the thickness of the encapsulant 18. Thinning of the encapsulant18 may be continued at least until the upper main surface of the viabars 11 is exposed.

By way of example, the encapsulant 18 in FIG. 1B may have a thickness ofabout a couple of hundred micrometers, e.g., more than 300 μm, 500 μm,800 μm or even more than 1000 μm. The thickness of the encapsulant 18 inFIG. 1B is greater than the thickness of the semiconductor chips 10. Assemiconductor wafers are often fabricated with a thickness of about 500μm or 1000 μm, and may be ground in frontend processes to be as small asabout 200 μm or even less, the thickness of the semiconductor chip 10may be, e.g., in a range of about 100 μm to 1000 μm. As an specificexample, the encapsulant 18 in FIG. 1B may have a thickness of about 800μm, the semiconductor chips 10 may have a thickness of about 250 μm andthe via bars 11 may have a thickness of about 300 μm. After thinning(FIG. 1C), the thickness of the encapsulant 18 may be reduced to about280 μm, and the upper surface of the encapsulant 18 may be planar.

FIGS. 1D to 1F illustrate method steps to produce conductiveredistribution structures 20, 30 on both main faces of the encapsulant18. First, the conductive redistribution structure 20 on the lower mainface of the encapsulant 18 and the lower main surface 12 of thesemiconductor chip 10 may be generated. These surfaces may be arrangedflush with each other, that is the conductive redistribution structure20 may be generated on a planar surface. The conductive redistributionstructure 20 may include a first polymer layer 21, a second polymerlayer 22 and a metal layer 23 arranged between the first polymer layer21 and the second polymer layer 22, see also FIG. 3.

The first polymer layer 21 may be deposited to cover the lower main faceof the encapsulant 18 and the lower main faces 12 of the semiconductorchips 10. The thickness of the first polymer layer 21 may be between 2and 10 μm, typically about 5 μm. A standard CVD (Chemical VaporDeposition) process or spin coating process may be used. The firstpolymer layer 21 may be made of a photoresist or of any other etchingresist.

The first polymer layer 21 is structured. Structuring may beaccomplished by photolithographic techniques known in the art. Duringstructuring, through-holes 21.1, 21.2 and 21.3 are generated in thefirst polymer layer 21. At the bottom of through-holes 21.1 and 21.2,the chip contact elements 14, 15 are exposed. At the bottom ofthrough-hole 21.3, the conductive element 11 b of the via bar 11 isexposed.

In a next step, the metal layer 23 is applied onto the first polymerlayer 21 and structured. In through-holes 21.1, 21.2 and 21.3, the metallayer 23 makes contact to the chip contact elements 14, 15 and theconductive element 11 b, respectively.

Many techniques are available to generate the structured metal layer 23,inter alia galvanic deposition, electroless deposition, printing etc.

The second polymer layer 22 is deposited over the metal layer 23. Thesecond polymer layer 22 may be made of the same material as the firstpolymer layer 21 and the thickness of the second polymer layer 22 may bein the same range as the thickness of the first polymer layer 21.

The second polymer layer 22 is then structured by, e.g.,photolithographic techniques to provide for openings 22.1, 22.2. Theexternal contact members 25, e.g., solder balls, are then applied (e.g.,so-called solder ball attach). Through the openings 22.1, 22.2, contactis made between external contact members 25 (e.g., solder balls) and themetal layer 23. The metal layer 23 connecting the contact elements 14,15 of the semiconductor chips 10 to the external contact members 25 ofthe semiconductor package 100 is often referred to as a redistributionlayer in the art.

The conductive redistribution structure 30 on the upper main face of theencapsulant 18 may be generated the same way as the conductiveredistribution structure 20. Similar to the lower main face 12, theupper main face of the encapsulant 18 may be planar. The conductiveredistribution structure 30 may include a metal layer 33 (correspondingto the metal layer 23) and a polymer layer 32 (corresponding to thesecond polymer layer 22) overlying the metal layer 33. The metal layer33 may be made of the same materials as and structured similar to themetal layer 23. The metal layer 33 includes structures which areelectrically connected (e.g., through via bars 11) to contact elements14, 15 of the semiconductor chip 10 and may form contact pads 50 of thesemiconductor package 100. The polymer layer 32 includes openings 32.1,32.2 through which the contact pads 50 may be electrically connected toexternal contact members (e.g., solder balls) 35 of anothersemiconductor package 300 to be mounted on semiconductor package 100 asshown in FIG. 3. The polymer layer 32 may optionally form a solder stoplayer which may be effective in a reflow process when mounting thesemiconductor package 300 on semiconductor package 100.

It is to be noted that the method steps used to generate the conductiveredistribution structures 20, 30 may be thin-film processing steps usingtechniques such as CVD, spin coating, galvanic plating, electrolessplating, printing, photolithography etc. These steps form part of theso-called backend fabrication process, i.e., are fabrication steps whichare applied after the integrated circuits of the semiconductor chips 10have been finished and tested on wafer level (so-called frontendprocessing). It is to be noted that the backend processing stepsillustrated in FIGS. 1D and 1E may still be performed on an artificialwafer level, i.e., before the separation of the artificial wafer intosingle semiconductor packages 100.

The separation of the artificial wafer into single semiconductorpackages 100 may be performed along dicing lines or dicing streets 60.Each semiconductor package 100 may contain one or more semiconductorchips 10. By way of example, separation may be performed by mechanicaldicing (e.g., blade sawing, cutting, water-jet separation), chemicaldicing (e.g., etching) or laser dicing.

FIGS. 2A to 2E schematically illustrate one embodiment of a method ofmanufacturing a semiconductor package 200. In a first step,semiconductor chips 10 are placed and fixed on a carrier 1 and areovermolded by an encapsulant 18. Optionally, the encapsulant 18 may thenbe thinned. In order to avoid reiteration, reference is made to thedescription of FIGS. 1A to 1C. The method steps used for generating theovermolded array of semiconductor chips 10 shown in FIG. 2A may onlydistinguish from the method steps shown in FIGS. 1A to 1C in that no viabars 11 are used.

The dielectric material of the encapsulant 18 may be structured asillustrated in FIG. 2B. A plurality of recesses 18.1, 18.2, 18.3 (orcutouts or through-holes or trenches) are created in the dielectricmaterial of the encapsulant 18 to expose at least portions of contactelements 14, 15, 16 of the semiconductor chip 10. The contact elements14, 15, 16 are contact pads of the semiconductor chip 10 and may belocated at the upper main surface of the semiconductor chip 10. Further,a recess 18.4 (or cutout or through-hole or trench) may be created in aregion outside of the outline of the semiconductor chip 10 and mayextend through the encapsulant 18 from one main face to the other mainface thereof. Removing the encapsulant 18 may be carried out by using alaser beam or a water jet, mechanical sawing using a saw or a cutter,chemical etching, milling or any other appropriate method. If theencapsulant 18 includes photo-active components, the encapsulant 18 mayalso be photo-lithographically structured. The widths of the recesses18.1, 18.2, 18.3 and 18.4 may, for example, be in the range from 10 to200 μm.

After structuring the dielectric material of the encapsulant 18, aconductive layer 40 is applied to the upper main face of the encapsulant18. The conductive layer 40 may consist of a seed layer (notillustrated) and a further layer which is galvanically deposited ontothe seed layer. The seed layer may consist of a barrier layer and astarting layer for the electroplating. An electroless deposition methodmay be used to produce the seed layer as well. The seed layer may have athickness of up to 1 μm and may, for example, be made of titanium,titanium containing alloy or chrome as barrier layer and, for example,copper as starting layer. The electrical conductivity of the seed layermay be used to galvanically deposit an electrically conductive layer,for example, a copper layer, on the seed layer. The copper layer mayhave virtually any desired thickness depending on the application andcurrent requirements. By way of example, the thickness of the copperlayer may be in a range between 2 μm and 15 μm. As an alternative to thegalvanic plating process describe above, an electroless plating processsuch as electroless nickel-palladium plating may be used.

The conductive layer 40 may also be created by depositing a pastecontaining fine metal particles in the recesses 18.1, 18.2, 18.3, 18.4and on the planar upper surface of the encapsulant 18, possibly afterapplication of a seed layer. The seed layer allows a low resistanceinterconnection to the aluminum of the contact pads of the chip. Themetal particles may, for example, be made of copper, silver, gold, tinor nickel or a metal alloy. The metal particles may be dispersed in asuitable liquid or solvent. The application of the paste containing themetal particles dispersed in the liquid may be performed by stencilprinting, screen printing, ink-jet printing or other suitable printingtechnologies. As illustrated in FIG. 2C, the conductive layer 40 is apatterned layer, wherein patterning may be performed during depositionof the paste. After the application of the paste, the paste may beexposed to an energy (e.g., elevated temperature, etc.). If elevatedtemperature is used, it may be in the range from 100 to 300° C. and inparticular in the range from 100 to 200° C. This temperature step causesthe liquid in the paste to evaporate. Furthermore, the appliedtemperature may be lower than the melting temperature of the metal (whenprovided in macroscopic dimensions) of which the metal particles aremade. Due to the temperature step, the metal particles may sinter andmay thus form a solid metal conductive layer 40. If a seed layer wasapplied prior to application of the metal particles it is removed bysuitable process steps, using the possibly sintered metal particles as amask for the seed layer.

Before or after the steps shown in FIGS. 2B and 2C, the encapsulant 18is released from the carrier 1. As illustrated in FIG. 2C, a bottomconductive layer 45 may be optionally applied to the planar lowersurface of the encapsulant 18. The bottom conductive layer 45 mayelectrically connect, e.g., through a conducting material filling recess18.4 to a contact element 16 of the semiconductor chip 10 at the uppersurface thereof. The bottom conductive layer 45 may be applied andstructured by the same processes and may consist of the same materialsas the upper conductive layer 40.

Similar to metal layer 33, the conductive layer 40 includes structureswhich are electrically connected to contact elements 14, 15 or 16 of thesemiconductor chip 10 and may form contact pads 50 of the semiconductorpackage 200. A polymer layer 42 may be applied over the structuredconductive layer 40, see FIG. 2D. The polymer layer 42 includes openings42.1, 42.2, 42.3 through which the contact pads 50 may be electricallyconnected to external contact members (e.g., solder balls) of anothersemiconductor package (e.g., semiconductor package 300 shown in FIG. 3)to be mounted on semiconductor package 200. The polymer layer 42 may beapplied similar to polymer layer 32 by thin-film technology processesand may (optionally) likewise form a solder stop layer as describedabove with reference to polymer layer 32. As to polymer layer 42, thedescription relating to polymer layer 32 is applicable and is referredto in order to avoid reiteration. Further, a polymer layer 43 may beapplied over the structured bottom conductive layer 45. The polymerlayer 43 may be applied and structured by the same processes and mayconsist of the same materials as the polymer layer 42. The polymer layer43 includes openings 43.1, . . . through which package terminals formedby the bottom conductive layer 40 and defined by the openings 43.1, . .. may be electrically connected to external contact members (e.g.,solder balls on a board).

The encapsulant 18 is then separated along dicing line 60 to obtainsingle semiconductor packages 200 (FIG. 2E). In this context, for thesake of brevity, reference is made to the description of FIG. 1F.

It is to be noted that packages 100, 200 are fan-out type packages.Contact pads 50 of fan-out type packages are located at least partiallyoutside of the lateral contour line of the semiconductor chip 10. Inanother embodiment (not shown), packages 100, 200 could be fabricated asfan-in type packages, in which the contact pads 50 are located insidethe lateral contour line of the semiconductor chip 10.

The semiconductor packages 100, 200, or any other semiconductor packageshaving contact pads 50 on the upper semiconductor package surface, maybe used for establishing package-on-package (PoP) devices. PoP devicestypically include two or more packages, which are stacked one on top ofthe other. Packages 100, 200 may be used as the lowermost or bottompackage.

PoP devices may be manufactured by the following process steps. First,the bottom semiconductor package 100, 200 is placed into pre-appliedsolder paste on a mounting platform, e.g., a board (not shown in FIG.3). Flux is applied to the contact members (e.g., solder balls) 35 ofthe upper or top semiconductor package 300, and the top semiconductorpackage 300 is then placed on the bottom semiconductor package 100, 200such that the contact members 35 of the top semiconductor package 300are aligned to contact pads 50 formed on the upper main face of thebottom semiconductor package 100, 200. During a reflow process, thispackage stack is then subjected to a temperature which is high enough toliquefy both contact members 25 and 35. The bottom semiconductor package100, 200 is mounted on the mounting platform such as, e.g., a PCB byreflow soldering. Simultaneously, the top semiconductor package 300 ismounted to the bottom semiconductor package 100, 200 by reflowsoldering. The assembly of PoP devices is referred to as second levelassembly in the art. It is often, but not necessarily, performed by acustomer of the manufacturer of the bottom semiconductor package 100,200.

The top semiconductor package 300 may be of various types and/ordesigns. It may include another semiconductor chip (not shown) and anencapsulant embedding this top package semiconductor chip. Further, itmay include top package contact pads (not shown) on a lower main face ofthe top semiconductor package 300, wherein the contact members 35 of thetop semiconductor package 300 are attached to these top package contactpads.

It is to be noted that when being placed on the bottom semiconductorpackage 100, 200, typically not all of the contact members 35 of the topsemiconductor package 300 will be in touch with the respective contactpad 50 beneath before the beginning of the reflow process. This is dueto inevitable warpage of the bottom semiconductor package 100, 200, thetop semiconductor package 300 or different sizes of the contact members35 (that is, e.g., variations in ball diameter). However, since thosecontact members (e.g., solder balls) 35 which are in touch with theirrespective contact pads 50 will collapse during the reflow operation,the top semiconductor package 300 will lower down by a certain extentduring the reflow operation, causing all contact members 35 to get intouch with their respective contact pads 50 beneath to establish secureelectrical contact in each case.

Substantial warpage of the bottom and/or top semiconductor package 100,200, 300 during reflow, however, may inhibit secure electrical contactbetween all of the contact members 35 of the top semiconductor package300 and the respective contact pads 50 of the bottom semiconductorpackage 100, 200. FIG. 4 illustrates a situation in which the bottomsemiconductor package 100, 200 and the top semiconductor package 300show warpage of the same type, that is warpage in the same direction,whereby, however, the warpage of the top semiconductor package 300 isgreater than the warpage of the bottom semiconductor package 100, 200.As a result, as the lower main face of the top semiconductor package 300is more curved than the upper main face of the bottom semiconductorpackage 100, 200, the gap between the two semiconductor packages 100,200, 300 has a greater spacing at edge regions of the semiconductorpackages 100, 300, 200 than at a central region of the semiconductorpackages 100, 300, 200. As a result, prior to the reflow operation, onlythe central contact member 35.1 (in practice, typically a number ofcentral contact members 35 which, however, are represented in FIG. 4 byone central contact member 35.1 for the sake of simplification) is intouch with the respective central contact pad 50 of the bottomsemiconductor package 100, 200. After the reflow operation, asillustrated in FIG. 4, although this contact member 35.1 has beencollapsed to a spacing S_(min), the spacing S_(max) at an edge region ofthe semiconductor packages 100, 300, 200 may still remain so large thatthe contact members 35.2, 35.3 at the edge region will not make contactto the respective contact pads 50 on the bottom semiconductor package100, 200 during the reflow operation. As a result, a faulty PoP deviceis assembled.

Similarly, contact failure may also occur if the bottom semiconductorpackage 100, 200 and the top semiconductor package 300 show warpage ofdifferent type (e.g. the top semiconductor package 300 shows warpage inthe other direction than the bottom semiconductor package 100, 200). Inthis case, the outermost contact members 35.2, 35.3 may provide secureelectrical contact with the respective contact pads 50 whilst thecentral contact member(s) 35.1 will not get in touch with the respectivecontact pad(s) 50 during the reflow process and will thus not establishelectrical contact(s).

It is to be noted that in practice, the type and degree of warpage ofthe top semiconductor package 300 is typically unknown to themanufacturer of the bottom semiconductor package 100, 200. By way ofexample, the top semiconductor package 300 may be a memory package,i.e., a semiconductor package embedding an integrated memory circuitsuch as, e.g., a RAM (random access memory). Although the functionalityof this integrated memory circuit is well-known to the manufacturer ofthe bottom semiconductor package 100, 200, and also some of the packagespecifications (e.g., the positions of the contact members 35) of thetop semiconductor package 300 are predetermined by the design of thebottom semiconductor package 100, 200 and are thus known to themanufacturer of the bottom semiconductor package 100, 200, the packagetype and/or the particular package design of the top semiconductorpackage 300 is typically unknown to the manufacturer of the bottomsemiconductor package 100, 200. By way of example, the top semiconductorpackage 300 may, e.g., be designed as a flip-chip BGA (Ball Grid Array),a BoC (Board on Chip) wirebonded type package or a WLP (Wafer LevelPackaging) fan-in or fan-out type package. These and other types anddesigns of packages exhibit quite different warpage behavior duringreflow soldering, but may all be available as top semiconductor packages300 for assembling PoP devices. That is, the manufacturer of the bottomsemiconductor package 100, 200 often has no influence on or knowledge ofwhich type or design of top semiconductor package 300 will be used(e.g., by a customer or even by the manufacturer itself for futuresecond level assembly). In other words, the manufacturer of the bottomsemiconductor package 100, 200 often may not predict the type and degreeof warpage of the top semiconductor package 300 to be used and thus thevariation of the spacing S between the stacked semiconductor packages100, 200 and 300.

According to one aspect, the geometry of the spacing S between thebottom semiconductor package 100, 200 and the top semiconductor package300 is influenced by the size of the exposed contact pad area of thecontact pads 50 at the upper main face of the bottom semiconductorpackage 100, 200. FIG. 5 illustrates the relationship between theexposed area of a contact pad 50 and the collapsing height H (overcontact pad 50) of a solder ball 35 attached to the contact pad 50. Theexposed contact pad areas are given by the diameters d1, d2 of theopenings 32.1, 32.2 of the polymer layer 32, respectively. The largerthe diameter d of the exposed contact pad area, the larger is the areawhich can be wetted by the liquid solder during reflow, and the lower isthen the height of the collapsed contact member 35, provided identicalsolder ball volumes are used. In other words, by increasing the diameterd of the exposed contact pad area, the spacing S between the bottom andtop semiconductor packages 100, 200, 300 may be decreased. In otherwords, the top semiconductor package 300 will be brought down nearer tothe bottom semiconductor package 100, 200 when the diameter d of theexposed contact pad area is increased, that is H2<H1 if d2>d1. By way ofexample, it has been found that an increase in diameter d from, e.g.,d1=240 μm to d2=260 μm may yield a decrease in the spacing S of about 8μm, that is H1−H2≈8 μm.

As stipulated in JEDEC, the Global Standards for the MicroelectronicsIndustry, specific package specifications should be met by bottomsemiconductor packages 100, 200 of PoP devices. In the following, theexposed contact pad area is also referred to as a “landing pad.” Typicallanding pad diameters d as of today are set out in Table 1.

TABLE 1 Typical landing pad Pad pitch Typical ball diameter diameter das of today 400 μm 250 μm 240 μm 500 μm 300 μm 280 μm 650 μm 350 μm 320μm . . .

According to one aspect, the diameter d in micrometer of an exposedcontact pad area of the contact pads 50 on the upper main face of thebottom semiconductor package 100, 200 is designed to satisfy therelationship

d≧(8/25)x+ 142 μm,   (1)

wherein x is the pitch of the contact pads 50 in micrometer (see alsothe illustration of x in FIGS. 7 and 8). Pitch x is also referred to as“ball pitch” in the art.

According to relationship (1), the diameter d of the exposed contact padarea is significantly larger than the diameter d set out in Table 1 andrecommended in the JEDEC standard. As a result of the increase of thediameter d of the exposed contact pad area, the collapsing height H ofthe contact members 35 is lowered during the reflow operation. Morespecifically, the difference between the two collapsed heights H1 (whenusing today's small diameter d1) and H2 (when using enhanced diameter d2according to relationship (1)) is the additional tolerance in warpageand contact member height (e.g., solder ball height), which can becompensated during second level assembly of the top semiconductorpackage 300.

The effect is illustrated in FIG. 6. In FIG. 6 the warpage of the bottomsemiconductor package 100, 200 and the top semiconductor package 300 isidentical to the warpage of these packages in FIG. 4. The diameter ofthe openings 32.1, 32.2 of the polymer layer 32 has been increased fromd1 to d2. As a result, the minimum spacing S_(min) between thesemiconductor packages 100, 200, 300 is reduced by H1-H2. The maximumspacing S_(max) is reduced by the same quantity H1-H2. As a consequence,the outermost contact members 35.2, 35.3 may come into contact with therespective contact pads 50 during the solder reflow operation.Therefore, as shown in FIG. 6, secure electrical contacts areestablished between all contact members 35.1, 35.2 and 35.3 and therespective contact pads 50, in particular, also at the edge regions ofthe stacked PoP device. The interconnect yield during second levelassembly is significantly improved, because larger warpage may becompensated.

The PoP device may have a spacing S between the lower main face of thetop semiconductor package 300 and the upper main face of the bottomsemiconductor package 100, 200 that varies by more than 8 μm, inparticular, 10 μm, more in particular, 12 μm across the lateralextension of the spacing S. Generally, warpage of the upper main face ofthe bottom semiconductor package 100, 200 and warpage of the lower mainface of the top semiconductor package 300 may both contribute to thevariation in spacing S of the PoP device. More specifically, the lowermain face of the top semiconductor package 300 may, e.g., have a warpageheight of at least 8 μm, in particular, 10 μm, more in particular, 12 μmacross the lateral extension of the lower main face. Further, the uppermain face of the bottom semiconductor package 100, 200 may, e.g., have awarpage height of at least 8 μm, in particular, 10 μm, more inparticular, 12 μm across the lateral extension of the upper main face.

This concept of increasing the diameter of the exposed contact pad areas50 on the upper main face of the bottom semiconductor package 100, 200is likewise applicable for any type of warpage behavior. By way ofexample, if the top semiconductor package 300 exhibits a warpage in thereverse direction as shown in FIGS. 4 and 6, the outermost contactmembers 35.2 and 35.3 are those contact members 35 which are initiallyin touch with the respective contact pads 50 beneath, and will collapsedown during reflow to a smaller height compared to the situation when astandard diameter of the exposed contact pad areas is used, resulting inthat the contact member(s) 35.1 in the central region of thesemiconductor package stack will then come in touch with the respectivecontact pad(s) 50 beneath and will thus provide secure electrical andmechanical contact therewith.

As known in the art, the TCoB (Temperature Cycling on Board) reliabilityof interconnect members (e.g., solder balls) will usually significantlyimpair on the reduction in height of these members. The reduction inTCoB reliability of interconnect members is caused by an increase ofshear stress, which occurs when the height of the interconnect membersbetween two bonded systems of a given difference of CTEs (Coefficient ofThermal Expansion) is reduced. As a matter of fact, the constraint ofsufficient TCoB reliability typically severely limits the possibility ofdecreasing the height of interconnect members between bonded systems.However, regarding second level assembly of PoP devices, the differencebetween the CTEs of two semiconductor packages is typically small, evenfor semiconductor packages of different type or design (e.g., BGA, eWLB,BoC). Therefore, when reducing the spacing S between the bottom and toppackages 100, 200, 300 and thus the height of the (collapsed) contactmembers 35, the TCoB-reliability is expected not to be significantlyreduced compared to conventional PoP devices using standard landing padareas as set out in Table 1.

FIG. 7 illustrates a sectional view of a bottom semiconductor package100 in which the diameter d of the exposed contact pad area is definedby the diameter of the openings 32.1, 32.2 of the polymer layer (solderstop layer) 32. This type of contact pad 50 is also referred to assolder mask defined contact pad.

A non-solder mask defined contact pad 50 is shown in FIG. 8. Here, thediameter d of the exposed area of the contact pad 50 is defined by thelateral dimensions of the metal layer 33 forming the contact pad, thatis by the shape of the metal layer 33 itself. By way of example, themetal layer 33 may exhibit a circular region 33.1 of diameter d, whichis connected to a conductor trace 33.2 of significantly smaller widththan d. The circular region 33.1 of the metal layer 33 then defines theexposed area of the non-solder mask defined contact pad 50. Throughoutthe specification and claims, the term “exposed contact pad area” andderivations thereof may relate both to a solder mask defined contact padand a non-solder mask defined contact pad.

In all embodiments, the exposed contact pad area must not be of circularshape but may be of any other suitable geometry, e.g., rectangular,polygonal etc. In this case, the meaning of d as used herein is toidentify an exposed contact pad area of size (π/4)d², i.e. a contact padhaving the same exposed area as a circular exposed contact pad area ofdiameter d. As can be seen in FIG. 8, in non-solder mask defined contactpads 50 the polymer layer 32 does not overly the metal layer 33 at thecircular region 33.1, and thus does not define the size of the exposedcontact pad area 50. Especially if non-solder mask defined contact pads50 are used (on the upper surface of the bottom semiconductor package100, 200 or on the lower surface of the top semiconductor package 300),TCoB-reliability may not be reduced compared to the standard approach.The interconnect yield is then improved without any decrease in theTCoB-reliability.

Possible diameters d of the exposed contact pad area are given in Table2. It is to be noted that the diameters d set out in Table 2 alwayssatisfy the relationship (1). More stringent conditions, which couldeven compensate for larger warpage of the semiconductor packages 100,200, 300, are given by:

d≧(8/25)x+152 μm,   (2)

or

d≧(8/25)x+162 μm.   (3)

Optionally, an upper limit of diameter d could be expressed by:

d≧(8/25)x +172 μm.   (4)

It is further noted that the relationships (1) to (4) may also apply forpitches x in 7between of the exemplary pitches of 400 μm, 500 μm, 650 μmset out in Table 2, and for pitches x smaller than 400 μm or larger than650 μm.

TABLE 2 Pad diameter d of exposed pad Pad pitch Typical ball diameterarea (landing pad diameter) 400 μm 250 μm 260-300 μm 500 μm 300 μm300-340 μm 650 μm 350 μm 340-380 μm . . .

By way of example, if the pitch is x=400 μm, the diameter d may satisfyd≧260 μm, in particular, d≧280 μm. If the pitch is x=500 μm, thediameter d may satisfy d≧300 μm, in particular, d≧320 μm. If the pitchis x =650 μm, the diameter d may satisfy d≧340 μm, in particular, d≧360μm.

In all embodiments, the solder material of the contact members 25, 35may, for example, be composed from at least one of the followingmaterials: SnPb, SnAg, SnAgCu, SnAgCuNi, SnAu, SnCu and SnBi.

In addition, while a particular feature or aspect of an embodiment ofthe invention may have been disclosed with respect to only one ofseveral implementations, such feature or aspect may be combined with oneor more other features or aspects of the other implementations as may bedesired and advantageous for any given or particular application.Furthermore, to the extent that the terms “include,” “have,” “with,” orother variants thereof are used in either the detailed description orthe claims, such terms are intended to be inclusive in a manner similarto the term “comprise.” Furthermore, it should be understood thatembodiments of the invention may be implemented in discrete circuits,partially integrated circuits or fully integrated circuits orprogramming means. Also, the term “exemplary” is merely meant as anexample, rather than the best or optimal. It is also to be appreciatedthat features and/or elements depicted herein are illustrated withparticular dimensions relative to one another for purposes of simplicityand ease of understanding, and that actual dimensions may differsubstantially from that illustrated herein.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A method of manufacturing a semiconductor package, the methodcomprising: embedding a semiconductor chip in an encapsulant; formingfirst contact pads on a first main face of the semiconductor package;and forming second contact pads on a second main face of thesemiconductor package opposite the first main face, wherein a diameter din micrometers of an exposed contact pad area of the second contact padssatisfies d≧(8/25)x+142 μm, wherein x is a pitch of the second contactpads in micrometers.
 2. The method of claim 1, further comprisingbonding the first contact pads to a board.
 3. The method of claim 1,further comprising bonding the second contact pads to anothersemiconductor package.
 4. The method of claim 3, wherein thesemiconductor chip comprises a logic circuit, a power circuit or anoptical circuit.
 5. The method of claim 4, wherein the anothersemiconductor package comprises a memory chip.
 6. The method of claim 3,wherein a spacing between the second main face of the semiconductorpackage and the another semiconductor package has a dimension thatvaries by more than eight micrometers across a lateral extension of thespacing.
 7. The method of claim 3, wherein a main face of the anothersemiconductor package facing the semiconductor package has a warpageheight of at least eight micrometers across a lateral extension of themain face.
 8. The method of claim 3, wherein the second main face of thesemiconductor package has a warpage height of at least eight micrometersacross a lateral extension of the second main face.
 9. The method ofclaim 1, wherein the exposed contact pad area of the second contact padsis defined by an opening in a solder mask layer overlying a metal layerforming the second contact pads.
 10. The method of claim 1, wherein theexposed contact pad area of the second contact pads is defined bylateral dimensions of a metal layer forming the second contact pads. 11.The method of claim 1, wherein the diameter d in micrometers satisfiesd≧(8/25)x+172 μm.
 12. The method of claim 1, wherein the pitch is x=400micrometers and the diameter is d 260 micrometers.
 13. The method ofclaim 1, wherein the pitch is x=500 micrometers and the diameter is d300 micrometers.
 14. The method of claim 1, wherein the pitch is x =650micrometers and the diameter is d 340 micrometers.
 15. The method ofclaim 1, further comprising: an element having a conductive via, theconductive via extending from the first main face of the semiconductorpackage to a metal layer forming the second contact pads.
 16. The methodof claim 15, wherein the element is made of an insulating material. 17.A method of manufacturing a semiconductor package, the methodcomprising: placing a plurality of semiconductor chips on a carrier;covering the semiconductor chips with a molding material to form anencapsulant; removing the carrier from the semiconductor chips; forminga number of contact pads on a surface of the encapsulant facing awayfrom the surface of the encapsulant which is defined by the carrier, adiameter d in micrometers of an exposed area of each contact padsatisfying d≧(8/25)x+142 μm, wherein x is a pitch of the contact pads inmicrometers; and singulating the semiconductor chips.
 18. The method ofclaim 17, further comprising: placing at least one element having aconductive via next to at least one of the semiconductor chips.
 19. Themethod of claim 17, wherein the diameter d in micrometers satisfiesd≧(8/25)x+172 μm.
 20. The method of claim 17, wherein the pitch is x=400micrometers and the diameter is d 260 micrometers.
 21. The method ofclaim 17, wherein the pitch is x=500 micrometers and the diameter isd≧300 micrometers.
 22. The method of claim 17, wherein the pitch isx=650 micrometers and the diameter is d 340 micrometers.
 23. A method ofmanufacturing a package-on-package device, the method comprising:providing a mounting platform; placing a first semiconductor packagecomprising a first semiconductor chip, an encapsulant embedding thefirst semiconductor chip, first contact pads on a first main face of thefirst semiconductor package and second contact pads on a second mainface of the first semiconductor package opposite to the first main faceon the mounting platform; placing a second semiconductor packagecomprising a second semiconductor chip, an encapsulant embedding thesecond semiconductor chip and third contact pads on a first main face ofthe second semiconductor package on the first semiconductor package,wherein the first main face of the second semiconductor package facesthe second main face of the first semiconductor package; and reflowsoldering to electrically connect the second contact pads to the thirdcontact pads and the first contact pads to the mounting platform,wherein a diameter d in micrometers of an exposed contact pad area ofthe second contact pads satisfies d≧(8/25)x+142 μm, wherein x is a pitchof the second contact pads in micrometers.
 24. The method of claim 23,wherein the semiconductor chip comprises a logic circuit, a powercircuit or an optical circuit and wherein the second semiconductorpackage comprises a memory chip.
 25. The method of claim 23, wherein aspacing between the second main face of the first semiconductor packageand the first main face of the second semiconductor package has adimension that varies by more than eight micrometers across a lateralextension of the spacing.